1. Field of the Invention
The present invention relates to a rotary driving apparatus that drives a rotating member to rotate, a control method therefor, a storage medium storing a control program therefor, and an image forming apparatus that has the rotary driving apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus rotates a rotating member, such as a photoconductive drum, at a constant rotational speed (angular velocity). For such a purpose, the apparatus rotates an encoder wheel on which graduations (scales) are marked at equal intervals with the rotating member in one piece, detects the rotating speed of the encoder wheel by detecting the graduations of the encoder wheel with an encoder sensor, and controls a motor that drives the rotating member based on the detected rotating speed.
If a dust particle adheres to the encoder wheel, the encoder sensor mistakes the dust particle for the graduation and outputs a signal (it is referred to as “abnormal periodic signal”, hereafter) that is different in a period from a signal corresponding to the actual rotational speed. When the rotation is controlled based on this abnormal periodic signal, it becomes impossible to rotate the rotating member at the constant rotational speed. There is a known method that generates a pseudo-internal signal (it is referred to as a “pseudo-signal”) when detecting an anomaly in the period of output signal and controls a motor based on the generated pseudo-signal (see Japanese Laid-Open Patent Publication (Kokai) No. 2005-28607 (JP 2005-28607A)).
The method disclosed in JP 2005-28607A is effective for the case where the period of output signal from the encoder sensor becomes abnormal in spite of the fact that the rotational speed of the rotating member is constant, such as a case where a dust particle adheres to the encoder wheel. However, when the rotational speed of the rotating member actually varies with a change of load etc., the variation in the rotational speed cannot be detected by the method disclosed in JP 2005-28607A, and it becomes impossible to rotate the rotating member at the desired rotational speed.